1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile device, a printer and the like, and also relates to a sheet feeding device arranged in the image forming apparatus for feeding a sheet-like recording medium (hereinafter referred to as sheet) to an image forming section.
2. Description of the Related Art
The sheet feeding device for feeding the sheet to an engine section that performs printing in an ink jet recording apparatus, one form of image forming apparatus, may adopt an auto sheet feeder (ASF) method or a paper feeding cassette method. In the ASF method, the sheet directly accommodated in the recording apparatus main body is fed, whereas in the paper feeding cassette method, a cassette is detachably attachable with respect to the recording apparatus main body with the sheets accommodated therein. A sheet feeding device combining the ASF method and the paper feeding cassette method is recently being put to practical use. In cases of large printer or copying machine, in particular, a model in which the sheet feeding devices having the same configuration are arranged in multiple levels to allow the sheets to be selectively fed is put to practical use.
With regards to the paper feeding cassette method, it is proposed that a sub-cassette is inserted above the main cassette to allow the sheets of different size to be selectively fed (Japanese Patent Application Laid-Open No. S61-188335). A configuration in which the sub-cassette is arranged overlapping the main cassette and feeding the sheet with separate paper feeding rollers is also proposed (Japanese Patent Application Laid-Open No. H06-40582). When such main cassette and sub-cassette are arranged, the plain papers such as A4 size are set in the main cassette and the thick L-size photo recording papers or post cards are set in the sub-cassette in many cases.
There are various types of sheets in recent years, and even printers and the like are able to respond to various printing. In particular, since the ink jet printer is a non-contact printing, the types of sheets are rarely limited, and thus the size and types of sheets are further increased, and the frequency of changing the sheets to be mounted accordingly becomes extremely high. A configuration in which a plurality of sheet feeding devices can be mounted so as to respond to sheets of different sizes even for a relatively low cost type printer is desired.
Behaviors exhibited by sheets during transportation also vary due to diversification. For instance, sheets such as ink jet post cards, glossy post cards, cards and business card size having a liquid ink receiving layer show the following behavior. Immediately after applying liquid ink during printing, some sheets greatly warp once and then curl, but such curl decreases and disappears with time and restores to its original shape. Since A4 sheets and the like having a size greater than the post card have a large sheet weight, large curls as seen in the sheet of small size are not produced. When curls are produced, the succeeding sheet may eject the previous first sheet together or the succeeding sheet may rub against the recorded surface of the previous sheet when ejecting the succeeding second sheet. In order to prevent such disadvantages, the step difference from the sheet ejecting port to the mounting surface of the ejected paper tray is sufficiently secured in the prior art even for small printers. However, with the ejected paper tray of a fixed configuration having the position of the sufficient step difference as a fixed position, many disadvantages arise in terms of high stacking performance of adapting to the size and type of the ejected sheet, and allowing as much sheet as possible to be mounted while aligning.
An apparatus in which the ejected paper tray is not fixed at the position having a step difference from the sheet ejecting port, and the ejected paper tray is moved in the up an down direction or in the vertical direction with driving means is proposed in order to achieve the ejected paper tray of high stacking performance (Japanese Patent Application Laid-Open No. H05-17064). Further, a configuration in which the plain cut papers are mounted on the main bottom plate of the ejected paper tray, and envelopes and the like are mounted on a sub-bottom plate arranged at one part of the main bottom plate to enhance the stacking performance is proposed (Japanese Patent Application Laid-Open No. 2002-87682). In this case, the main bottom plate is supported by a spring from below, and the main bottom plate elastically compresses the spring and sinks in response to the weight of the sheet. Since the sub-bottom plate is for small envelopes, the spring force of the spring for the sub-bottom plate is large compared to that of the spring for the main bottom plate. Therefore, in the case of envelopes, the spring is compressed by way of the sub-bottom plate after a greater number of papers than the plain cut papers are mounted. Thus, the stacking performance is enhanced even in the case of small envelopes having a sheet thickness of a few times greater than the plain cut paper by mounting a great number of papers.
However, the prior arts have the following problems. The sheet feeding device disclosed in Japanese Patent Application Laid-Open No. S61-188335 is troublesome in that the cassette must be changed manually, and the sub-cassette must be taken out and stored each time when using the main cassette.
Further, in the case of the sheet feeding device disclosed in Japanese Patent Application Laid-Open No. H06-40582, the occupying space of the sheet feeding device becomes extremely large with respect to the printer main body.
Further, with regards to the mounting tray disclosed in Japanese Patent Application Laid-Open No. 2002-87682, if the step difference from the sheet ejecting port to the mounting surface of the ejected paper tray is sufficiently secured so as to correspond to various sheets, the device becomes larger by such amount. In addition, driving means for moving the ejected paper tray up and down is arranged, whereby the device becomes larger and more complicating with increase in the driving motor and the drive transmission means.
Moreover, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H05-17064, the position in the up and down direction of the main bottom plate and the sub-bottom plate is determined by balancing the spring force of the spring and the sheet mounted weight. Therefore, the sheet mounting amount at the main bottom plate and the sub-bottom plate is not guaranteed if the spring properties of the springs of the main bottom plate and the sub-bottom plate vary. In addition, diversification of the sheet type cannot be responded with a unique spring force.